1. Field of the Invention
The present invention relates to a stack having a Heusler alloy, a magnetoresistive element and spin transistor using the stack, and a method of manufacturing the same.
2. Description of the Related Art
Recently, a magnetic memory device (magnetic random access memory [MRAM]) using, as a memory element, a magnetic tunnel junction (MTJ) element having a magnetic tunnel junction formed by a stacked structure of a ferromagnetic material layer/insulator layer (tunnel barrier)/ferromagnetic material layer has been proposed.
In this MRAM, the resistance of the stacked structure is changed by fixing magnetization direction (or majority spin axis) in one ferromagnetic material layer (a reference layer or fixed layer) and controlling magnetization direction (or majority spin axis) in the other ferromagnetic material layer (a recording layer or free layer), thereby storing a high-resistance state or low-resistance state as data “0” or “1”. For example, the resistance is low when the spins in the reference layer and recording layer are parallel, and high when they are antiparallel.
The magnetoresistance ratio (MR ratio) of this MTJ element was a few 10% at room temperature a few years ago, but has reached 500% in recent years. This extends the possibility as various spin devices in addition to the MRAM. An example is a spin MOS field-effect transistor (spin MOSFET).
Although the MR ratio has increased as described above, it is necessary to further increase the MR ratio in order to achieve a high-efficiency magnetic memory device or spin MOSFET. Especially when applying the MTJ to a semiconductor device such as the MRAM or spin MOSFET, it is necessary to obtain a high MR ratio in a resistance region where a sheet resistance RA is about 10 Ωμm2. When the thickness of the tunnel barrier of an MTJ element by which an MR ratio of 500% is obtained is decreased in order to obtain RA=10 Ωμm2, the MR ratio decreases to 200%.
A promising approach for solving this problem is to use a ferromagnetic material having a high spin polarization (P) as the ferromagnetic material layer of the MTJ. According to the Julliere's law, the MR ratio is theoretically infinite if a half-metal material having P=100% is used. Candidates of this half-metal material are, e.g., CrO2, Fe3O4, and a Heusler alloy. Recently, a Co-based Heusler alloy has achieved a high MR ratio, and spin devices using these materials are expected.
Furthermore, a device structure combining the Co-based Heusler alloy and a tunnel barrier made of magnesium oxide (MgO) in the MTJ is recently attracting attention (see, e.g., N. Tezuka, et al., Appl. Phys. Lett. 89 (2006) 112514). This combination of the Heusler alloy and MgO tunnel barrier is expected to be applied not only to the MTJ but also to the source/drain of the spin MOSFET.
Note that the Heusler alloy (also called a full-Heusler alloy) is a general term of intermetallic compounds having a chemical composition represented by X2YZ. X is Co-, Fe-, Ni-, or Cu-based transition metal element or noble metal element on the periodic table. Y is an Mn-, V-, Cr-, or Ti-based transition metal, and can be the same element species as X. Z is a typical element of groups III to V. The Heusler alloy X2YZ can be classified into three types of crystal structures in accordance with the regularity of X.Y.Z. A structure having highest regularity in which the three elements can be distinguished from each other like X≠Y≠Z by analysis such as X-ray diffraction using the periodicity of a crystal is the L21 structure. A structure having second highest regularity in which X≠Y=Z is the B2 structure. A structure in which the three elements cannot be distinguished from each like X=Y=Z is the A2 structure.
To control a spin-polarized current by the gate voltage in the spin MOSFET, it is important to inject a current (to be also referred to as a highly spin-polarized current hereinafter) having a highly spin-polarized electron ratio into the channel from a magnetic material layer in the source. Also, in the spin MOSFET and MTJ, the magnetoresistive (MR) effect controlled by the relative magnetization directions in two magnetic material layers sandwiching a nonmagnetic layer is the basic operation principle of the device.
When performing spin transfer in a write method using spin transfer in the spin MOSFET and MTJ, no spin reversal occurs unless a current having a very high current density is supplied to the element. When a current having a high current density is supplied to a magnetoresistive effect element having a tunnel barrier layer, the element breaks because a high electric field is applied to the tunnel barrier. Therefore, a structure in which spin reversal occurs by a current having a low current density is required. Theoretically, a high spin polarization (P) is important for spin transfer as well. Furthermore, a Heusler alloy generally has small saturation magnetization and a small damping constant, and hence is a material advantageous for spin-transfer torque magnetization reversal.
As explained above, in the magnetoresistive effect element, magnetic memory device, and spin MOSFET, generation and injection of a highly spin-polarized current are essential to implement the device and improve its performance. One promising solution is an electrode structure combining a Heusler alloy and crystalline tunnel barrier.
Unfortunately, the spin polarization of a current realized by this structure strongly depends on the crystal regularity of a Heusler alloy. Theoretically, even a Heusler alloy exhibiting half-metal characteristics by the L21 structure takes the A2 structure depending on the formation conditions, and becomes the same as a normal ferromagnetic material. Also, to extract the properties as a magnetoresistance, a Heusler alloy must have the L21 structure in the interface in contact with a tunnel insulating film.
As a method of increasing the regularity of a Heusler alloy, a method of epitaxially growing a Heusler alloy on a substrate or buffering layer having high lattice matching with the Heusler alloy and annealing the Heusler alloy is known. That is, it is impossible to extract high performance from a device using a Heusler alloy without epitaxially growing it.
Unfortunately, this method has the problems that, e.g., the degree of freedom of selection of an underlying layer for forming a Heusler alloy is low, a high annealing temperature is necessary, and the number of times of annealing increases. Since most magnetic devices are formed on semiconductor integrated circuits, these technical problems interfere with device applications.